Air carrier has TCASII TA then RA but no target showing. RA continues. Evasive action, descend.

Attributes
ACN	241015
Time
Date	199304
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : isp
State Reference	NY
Altitude	msl bound lower : 2500 msl bound upper : 3000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	tracon : n90
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Make Model Name	Small Transport
Flight Phase	cruise other
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 20 flight time total : 12000 flight time type : 700
ASRS Report	241015
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : atp pilot : commercial
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance
Independent Detector	aircraft equipment other aircraft equipment : unspecified
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 1500 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	other

Narrative:

Received TA, saw target in instrument (TCASII) at 6 O'clock, '0' altitude, followed by RA to descend. No corresponding target. Hesitated to follow command. Advisory intensified and descent was made until clear. ATC was advised. Descended 500-600 ft off assigned altitude. Saw a light twin pass left to right above. TCASII then advised clear of conflict. ATC confirmed target, however, did not inform us of either conflict's presence.

Original NASA ASRS Text

Title: ACR HAS TCASII TA THEN RA BUT NO TARGET SHOWING. RA CONTINUES. EVASIVE ACTION, DSND.

Narrative: RECEIVED TA, SAW TARGET IN INST (TCASII) AT 6 O'CLOCK, '0' ALT, FOLLOWED BY RA TO DSND. NO CORRESPONDING TARGET. HESITATED TO FOLLOW COMMAND. ADVISORY INTENSIFIED AND DSCNT WAS MADE UNTIL CLR. ATC WAS ADVISED. DSNDED 500-600 FT OFF ASSIGNED ALT. SAW A LIGHT TWIN PASS L TO R ABOVE. TCASII THEN ADVISED CLR OF CONFLICT. ATC CONFIRMED TARGET, HOWEVER, DID NOT INFORM US OF EITHER CONFLICT'S PRESENCE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.